HE A T APPARA TVS. 1 3 1 



The direct verification of the fact that heat disappears when 

 work is done by a heat-engine, unsuccessfully attempted by 

 Seguin in 1839, was first effected by Him in 1857. 



A great variety of indirect methods of approximating to the 

 mechanical equivalent of heat have been successfully applied 

 within the last thirty years by different experimenters. The 

 earliest are, of course, those of Joule effected in 1843, and subse- 

 quent years by means of magneto-electricity. 



The results of all such experiments are briefly summed up in 

 the statement known as the. 



FIRST LAW OF THERMODYNAMICS. 



When equal quantities of mechanical effect are produced by any 

 means from purely thermal sources, or lost in purely thermal effects, 

 equal quantities of heat are put out of existence or are generated. And, 

 in the latitude of Manchester, 772 foot-pounds of work are capable 

 of raising the temperature of lib. of water from 50 F. to 51 F. 



Perhaps no purely physical idea has done so much to simplify 

 science, or led to so many singular and novel predictions (subse- 

 quently verified by experiment) as has Carnot's idea of a Cycle, or 

 his farther idea of a Reversible Cycle of operations. 



It has given us not only the legitimate mode of finding the rela- 

 tion between heat and work in an engine, but also the test of per- 

 fection for a heat-engine, an absolute definition of temperature, the 

 effect of pressure on the melting points of solids, and innumerable 

 important groups of associated properties of matter and energy 

 under various conditions. To a great extent these are included 

 in the statement of the 



SECOND LAW OF THERMODYNAMICS. 



If an engine be such that, when it is worked backwards, the phy- 

 sical and mechanical agencies in every part of its motions are all 

 reversed, it produces as much mechanical effect as can be produced by 



K 2 



